1. Field of the Invention
This invention relates to novel binders containing NCO and polyortho ester groups, to a process for preparing them and to their use.
2. Description of the Prior Art
Polyortho esters come about through the transesterification of acyclic ortho estes with polyfunctional alcohols. Since formation of polyortho ester under acid catalysis in an aqueous medium is reversible, (poly)ortho ester groups can be considered latent or masked OH groups. Through hydrolysis the OH groups are released so that the acyclic ortho ester initially used for masking is not entirely eliminated, therefore remaining in the polymer. This system, accordingly, represents a latent OH compound free from elimination products.
EP-B 882 106 describes coating materials based on crosslinkers containing bicyclic ortho ester groups as latent OH groups. Hydrolysis and crosslinking of these bicyclic ortho ester groups is accomplished in accordance with the same principle as in the case of the polyortho esters. Coating materials of this kind result in fast-drying films possessing excellent chemical resistance.
DE-A 31 03 779 and DE-A 31 28 217 describe stoichiometric adducts of OH-containing bicyclic ortho esters with polyisocyanates, giving polyurethanes containing bicyclic ortho ester groups as reactive functions. These functions can then be crosslinked with one another via cationic ring-opening homopolymerization in the absence of atmospheric moisture.
WO 99/10397 A1 discloses compounds which in addition to bicyclic ortho ester groups also have free NCO groups, so that they can be cured self-crosslinkingly following deblocking of the latent OH groups.
Countering their very rapid drying rate on crosslinking with isocyanates, however, bicyclic ortho ester groups as latent OH groups have the disadvantage that their preparation and incorporation into relatively high molecular mass compounds, which are then suitable for producing paints and plastics, involves multistage syntheses.
DE-A 26 25 392 and EP-A 1 225 172 describe the blocking of polyols with acyclic ortho esters and the use of these masked polyols in combination with polyisocyanates to prepare polyurethanes. The deblocking of the OH groups takes place in this case following application, as a result of the effect of atmospheric moisture. As compared with conventional 2K (two-component) PU systems based on polyacrylates and aliphatic polyisocyanates, however, there is no rate advantage attaching to the crosslinking.
For automotive refinish materials the demand is for fast-drying paints, and particularly paints based on linear-aliphatic isocyanates. The reason for this lies in their good film properties such as, for example, good mechanical and chemical resistance, good film optical qualities, and efficient processing (few spray passes, fast drying). In order to increase productivity a primary requirement is for even faster drying than has been possible to date using 2K PU coating materials. Additionally, for reasons of application reliability, very long pot lives are required.